Last year, the boys did very well with watercolor, so I decided to introduce it earlier this year and maybe we'll do more than one.
I like to teach the boys watercolor step-by-step. So I'm not really teaching them concepts. I think they'd get bored with that. Right now it's all about results. So we just take it one thing at a time. We're painting a sky. Then painting some bushes. Etc. I try to encourage them to use different colors, paint wet-on-wet, and other things of the like, but I also find that I can only throw one or two things at them before it becomes too much. And even then, they are usually so gung-ho to get started that I only have half their attention.
Here's Magoo working on the shrubs at the horizon line.
Here are the finished paintings. Didn't they do a good job?
I worked from the book The Watercolor Painting Book by Wendon Blake which is a great book for step-by-step landscapes. It's out-of-print but it looks like Amazon has a few left. It's not a children's art book, it's just a standard watercolor book. It's not as glossy as a new watercolor book, but it really has some sound lessons in it and I find it's one of the books I refer to the most.